Field of the Invention
The present invention relates to an electrode material, a paste, an electrode plate, and a lithium ion battery, particularly, to an electrode material which is suitably used for a cathode of a lithium ion battery, a paste, an electrode plate, and a lithium ion battery including this electrode plate.
Description of Related Art
Recently, along with the rapid progress in the development of clean energy techniques, the development of techniques aimed at earth-friendly societies has been progressing, for example, the wide use of post-petroleum, zero-emission, and power-saving products. In particular, recently, secondary batteries which are used for high-capacity storage batteries, portable electronic equipment, or the like and can supply energy for electric vehicles or in cases of emergency have been in the limelight. As such secondary batteries, for example, lead storage batteries, alkali storage batteries, or lithium ion batteries are known.
In particular, lithium ion batteries which are non-aqueous electrolytic solution secondary batteries can be reduced in size and weight and increased in capacity and have superior properties such as high output and high energy density. Therefore, lithium ion batteries have been commercialized as a high-output power supply of electric vehicles, electric tools, or the like, and next-generation lithium ion battery materials have been actively developed all over the world.
In addition, recently, a home energy management system (HEMS) which is a collaboration of energy techniques and housing techniques has been known. A smart energy-saving system has attracted attention in which the optimization of automatic control, electric power supply and demand, and the like is controlled by integrating information relating to home electric appliances such as smart appliances, electric vehicles, or photovoltaic power generators and control system thereof.
As a cathode active material for a lithium ion battery which has been into practice, LiCoO2 or LiMnO2 is commonly used. However, Co is a rare resource which is unevenly distributed on earth and thus, for example, when being required to be used in a large amount as a cathode material, has a problem in that the production cost of a product is increased and stable supply is difficult. As an alternative cathode active material to LiCoO2, the research and development of a cathode active material such as LiMn2O4 having a spinel crystal structure, LiNi1/3Mn1/3Co1/3O2 having a ternary system composition, lithium iron oxide (LiFeO2) which is an iron-based compound, or lithium iron phosphate (LiFePO4) and lithium manganese phosphate (LiMnPO4) which have an olivine structure have been actively progressed.
In such cathode active materials having an olivine structure, since the electron conductivity is insufficient, various countermeasures such as the refining of particles or the forming of a composite material between particles and a conductive material are required for high-current charging and discharging, and much effort has been made to solve this problem.
However, when particles are refined or when a large amount of conductive material is used to form a composite material, the electrode density is decreased, which leads to a decrease in battery density. That is, there is a problem in that the capacity per unit volume is decreased. As a method for solving this problem, a carbon coating method is disclosed, the carbon coating method including: mixing an organic material solution, which is an electron conductive material and a carbon precursor, with electrode active material particles to obtain a mixture; drying the mixture to obtain a dry material; heating the dry material in a non-oxidizing atmosphere to carbonize an organic material such that an electrode material in which surfaces of the electrode active material particles are coated with carbon is obtained.
This carbon coating method has advantageous effects in that the surfaces of the electrode active material particles can be coated with carbon, and the conductivity can be improved without a significant decrease in electrode density. Therefore, various techniques relating to this method have been disclosed.
As one of these techniques, an electrode material in which surfaces of LiFePO4 particles are coated with carbon produced by thermal decomposition of reducing sugar is disclosed (for example, refer to Japanese Laid-Open Patent Publication No. 2009-190957).
This electrode material can be easily synthesized by spraying a solution or a suspension including an Fe component, a P component, and reducing sugar and heating the solution or the suspension.
However, most of electrode materials which are obtained with the carbon coating method are LiFePO4 alone or a compound containing LiFePO4 and a slight amount of a different element. Therefore, there is a problem in that carbon active materials having an olivine structure other than LiFePO4 are not sufficiently coated with carbon.
For example, in the case of LiMnPO4, since Mn functions as a negative catalyst for suppressing a carbonization reaction, it is difficult to improve the electron conductivity with the carbon coating method.
Since LiFePO4 has lower capacity and energy density than those of other cathode active materials having an olivine structure such as LiMnPO4, a carbon coating method is also required for carbon active materials having an olivine structure other than LiFePO4.
In order to solve the above-described problems, a method of improving a coverage of carbon in LiMnPO4 by allowing nickel or iron to be present on surfaces of particles is disclosed (for example, refer to Japanese Laid-Open Patent Publication No. 2010-135305).